This application relates, in general, to a plastic material often used in high pressure applications having improved flexibility and a method of manufacturing such material, and more particularly to the addition of a polymer modifier to a base polymer for increasing the flexibility of a plastic material which does not deleteriously affect the base polymer structure and/or strength of the plastic material.
A typical technique for increasing the flexibility of plastic parts is through the addition to the base resin of a relatively lower density polymer or a more amorphous polymer, such as a plastomer, to form the plastic part. Currently used plastomers, while effectively increasing the flexibility of the product, have the disadvantage of decreasing the strength of the plastic as it can deleteriously affect the base polymer structure of the plastic material. In particular, the plastomer typically decreases the strength of the plastic material due to the more amorphous structure.
Crosslinked polyethylene, sometimes referred to as PEX or XL-PE, is a well known plastic material having many common uses. PEX is commonly used in the production of tubing, conduits, and piping for use in a variety of applications such as fire protection, plumbing, heating, gas distribution, and the like. Due to the flexibility and strength of PEX at temperatures ranging from below freezing up to 93° C. (200° F.), PEX is an ideal piping material for hot and cold water plumbing systems, hydronic radiant heating systems, snow melting applications, ice rinks and refrigeration warehouses.
A common use of PEX is in the production of barrier pipes. Barrier pipes are plastic water pipes that are used in domestic heating systems. The pipe is manufactured with a barrier that prevents oxygen from penetrating the material and entering the water system, reducing the risk of corrosion. The oxygen barrier layer is usually a resin material bonded between the outer and inner layer of the pipe itself.
Another common use of crosslinked polyethylene is in wire and cable applications including coatings such as, for example, insulation or jacketing.
In the production of PEX, crosslinks between polyethylene macromolecules are formed to make the resulting molecule more durable under temperature extremes and chemical attack, and more resistant to creep deformation. It is noted that in highly filled blends, such as for wire and cable applications, for example, that the PE is not necessarily considered to strictly crosslink. Therefore, for the purpose of this invention, filled systems that couple, bond or graft, will be considered crosslinked for purposes of the description herein and included within the definition of crosslinked polyethylene.
When attempting to increase the flexibility of PEX pipes for certain uses, such as barrier pipes, to reach a particular flexibility goal, the load levels of previously used plastomers had to be high, usually in excess of 20 percent by weight of the total material composition. These high loadings of plastomer diluted the base PEX polymer structure. Consequently, the finished pipes have poor pressure holding capability, typically fail the required pressure tests and often have impaired high temperature properties.
Semi-crystalline materials such as, for example, polyolefins, are characterized as having an amorphous phase and a crystalline phase. Much of their properties are derived from the amount and morphology of these two phases. Hardness and strength, as examples, are increased with increasing crystallinity whereas flexibility and toughness, as examples, are increased with decreasing crystallinity. This is generally true for high-crystalline materials like plastics, intermediate materials like plastomers and low-crystalline materials like elastomers or rubbers.
In many semi-crystalline materials, and particularly in semi-crystalline polyolefin plastics, the strength and hardness arise from the crystalline phase of the polymer. The crystallinity acts as hard-block crosslink points with interconnecting chains. The overall network formed resists deformation on strain. In plastics this results in high hardness and improved strength. The flexibility and toughness of the semi-crystalline polyolefin arises from the amorphous phase where the chains are entangled randomly. Freedom of the entangled chains to move provides a mechanism for the polymer to absorb impact and flex. There is a balance of desired properties in many polymer applications where better toughness or flexibility is achieved by reducing the crystallinity. However, lowering crystallinity reduces the strength and hardness of the polymer. Conversely, stronger, harder semi-crystalline materials are achieved by increasing the crystallinity at the expense of toughness and flexibility.
One way to extend this balance of properties and increase the strength or hardness without sacrificing toughness or flexibility is to crosslink the chains in the amorphous portion of the polymer. This creates a higher crosslink network density without increasing the crystallinity or hardness of the polyethylene. In ethylene homopolymers and copolymers the chains can be crosslinked in a number of ways including the free-radical chemistry of peroxides, silane chemistry, radical formation from high-energy radiation such as e-beams, and other methods.
To further affect the balance of properties, the addition of modifiers or plasticizers to the polymer is often used to soften the material and improve flexibility. It is understood that the modifiers need to be compatible with the host polymer and that they are generally excluded from the crystalline phase and reside predominantly in the amorphous phase of the host polymer. Typical modifiers can be high Mw, low density copolymers such as plastomers, reactor copolymers (R-COPO's), as well as low molecular weight fluid modifiers like mineral oil, white oil and paraffinic oils.
However, as the semi-crystalline polyolefins become more crystalline and plastic-like, it becomes more difficult to modify them. One reason for this is that there is much less amorphous phase for the modifier to occupy and another is that the compatibility with the host polymer often becomes low.
One way previously used to modify the properties of PE, including HD, metallocene, LD and LL in both thermoplastic blends or PEX, was to add lower density ethylene copolymers such as reactor copolymer (R-COPO) or ethylene butene (EB) or ethylene octene (EO) plastomers. When added to HDPE, these materials are incompatible and often form separate rubber domains within the amorphous phase of the host polymer. The resulting two phase morphology can provide impact resistance, in some cases, but these high Mw polymer modifiers also bring undesirable properties such as poor processing, loss of crosslinking efficiency and reduced toughness. They are often more difficult to blend, have reduced cut-through resistance, require a fine control of morphology and often have compatibility issues with the host polymer. In addition, adding a softer polymer modifier will generally reduce the tensile properties, but in an ineffective way. Often it requires a significant amount of rubber modifier to make a tensile property change in the host polymer. Overall, using rubber modifiers to improve flexibility is ineffective.
Another way to modify the properties of thermoplastic PE or PEX is to add fluid modifiers such as mineral oils, white oils and paraffinic oils. However, one problem encountered when using typical plasticizers in crosslinked polyolefin applications is that they act to reduce the efficiency of the cure systems. To counter this effect one must either limit the amount of fluid modifier used, or increase the amount of curative used to achieve the desired crosslink density and physical properties.
Another problem encountered with typical mineral oil modifiers is the compatibility with the host polymer. Since typical modifiers have broad molecular weight distribution (MWD) and a complex composition there are polar components and low molecular weight species that bloom to the surface of the host polymer. As the modifier migrates to the surface, its concentration in the host polymer is reduced over time, and the polymer properties can change significantly. In this example the modifier is said to have low permanence.
DE 1769723 discloses crosslinked PE compositions blended with various oils, apparently for use in electrical cables.
WO 2004/014988, WO 2004/014997, US 2004/0054040, US 2005/0148720, US 2004/0260001, US 2004/0186214, disclose blends of various polyolefins with non-functionalized plasticizers for multiple uses.
US 2006/0247331 and US 2006/0008643 disclose blends of polypropylene and non-functionalized plasticizers for multiple uses.
WO 2006/083540 discloses blends of polyethylene and non-functionalized plasticizers, but do not show specific blends of PE-X and non-functionalized plasticizers for multiple uses.
Silane crosslinked polyethylene for wire and cable applications is disclosed in U.S. Pat. No. 7,153,571.
Other references of interest include U.S. 2001/0056159, U.S. Pat. No. 5,728,754, EP 030 757076 A1, EP 0 755 970 A1, U.S. Pat. Nos. 5,494,962, 5,162,436, EP 0 407 098 B1, EP 0 404 011 A2, EP 0 344 014 A2, U.S. Pat. Nos. 3,415,925, 4,536,537, 4,774,277, JP 56095938 A, EP 0 046 536 B1, and EP 0 448 259 B1.